Monocrystalline material is used in the manufacture of a wide variety of products, such as integrated circuits, optical systems, and various other microminiature devices. There are a number of methods that are utilized in the growth of a monocrystalline structure from a melt. A synopsis of some of the more significant methods is given below.
The Czochralski method involves the melting of a polycrystalline charge in a crucible by radio frequency induction or resistance heating. A monocrystalline seed is then lowered into the melt while being rotated in, e.g., a clockwise direction. The crucible and its charge, if desirable, can be rotated in a counterclockwise direction. The seed crystal is withdrawn at a slow rate from the melt until the desired diameter of the pulled monocrystalline structure is obtained. The pull speed is adjusted to maintain the desired diameter of the pulled structure. This procedure continues as long as there is melt remaining in the crucible. One problem encountered with the Czochralski method is that of controlling the cross-sectional area of the crystal. Although a circular cross-section is produced, the diameter of the crystal has a tendency to vary widely as the growth proceeds. An additional disadvantage is that the monocrystalline structure pulled from the charge may be contaminated by the material of the crucible. Furthermore, a dopant, which is intentionally added to control the properties of the crystal to be grown, may be unevenly and unpredictably distributed through the crystal, causing non-uniformity in physical properties. The concentration of dopant in the bulk melt tends to increase as the crystal grows because dopants and other impurities tend to be rejected by the growth front back to the bulk melt. Examples of dopants are P and B in n and p-type Si semiconductors, respectively, and Nd in YAG (yttrium aluminum garnet) lasers.
The Stepanov method (See A. V. Stepanov, Bull. Acad. Sci. USSR, Vol. 33, 1969, p. 1775) is a modification of the conventional Czochralski method. In the Stepanov method, a die member is mounted at a fixed position within the crucible such that the upper edges of the die are above the surface of the melt and the bottom of the die is well below the surface of the melt. The key to Stephanov's technique is shaping a melt column which is used to control the crystal shape. See H. E. LaBelle, Jr., J. Crystal Growth, Vol. 50, 1980, p. 8. A difficulty with the Stepanov method is that constant control of the melt level in the crucible is required, because the level of the melt and hence the shape of the melt column will vary upon formation of the crystal.
The Stephanov method has been used for Ge, Si, and InSb, and graphite shapers are usually used. Limitations of this method are that careful adjustments must be made to keep the shaper top even with the melt level at all times. In general, the melt must not wet the shaper.
For growth of thin sheets, thin tubes, ribbons and fibers of sapphire, an edge-defined film fed growth method (EDFG) is disclosed by La Belle, Jr., in, e.g., U.S. Pat. Nos. 3,471,266; 3,527,574; 3,846,082, and 3,915,662. A fixed capillary tube is used as a shaper. No thermocouple is attached to the shaper. It is necessary for the melt to wet the shaper, which causes it to sink if it is in the form of a thin washer. One problem with the method is that cross-sections of the crystals are prohibitively small. The cross-section of the crystal decreases as the melt level drops.
In the edge-defined, film-fed growth (EDFG) technique, the shape of the crystalline body is determined by the external or edge configuration of the end surface of a forming member or die. An advantage of the process is that bodies of selected shapes such as round tubes or flat ribbons can be produced. The process involves growth on a seed from a liquid film of feed material sandwiched between the growing body and the end surface of the die, with the liquid in the film being continuously replenished from a suitable melt reservoir via one or more capillaries in the die member. By appropriately controlling the pulling speed of the growing body and the temperature of the liquid, the film can be made to spread (under the influence of the surface tension at its periphery) across the full expanse of the end surface of the die until it reaches the perimeter or perimeters thereof formed by intersection of that surface with the side surface or surfaces of the die. The angle of intersection of the aforesaid surfaces of the die relative to the contact angle of the liquid film is such that the liquid's surface tension will prevent it from overrunning the edge or edges of the die's end surface. The growing body grows to the shape of the film which conforms to the edge configuration of the die's end surface.
The Bridgman-Stockbarger method utilizes an elongated container of material which is melted in a high temperature furnace, after which the container is lowered into a cooler, lower temperature furnace, which allows the material to slowly resolidify as a single crystal. The molten material is in contact with the container wall during the process, and as a result, strains occur in the material which induce defects when the molten material solidifies.
Float zone refining is another method used to convert polycrystalline material to a high quality monocrystalline rod and, simultaneously, to remove unwanted impurities from the material. In the float zone technique a narrow molten zone is caused to move slowly along the length of a vertically disposed rod of polycrystalline material. As the molten zone moves, the material immediately behind the zone resolidifies as monocrystalline material. The monocrystalline growth is initially nucleated by a single crystal seed and then continues in a self-seeding manner. Impurities in the material tend to congregate in front of the molten zone so that as the molten zones moves, the zone also removes impurities with it, leaving the material behind the zone in a purer state.
In the float zone process with a contactless heater, the molten zone is caused to transverse the length of the polycrystalline rod by moving the rod vertically downward past a stationary heating means such as a radio frequency induction coil that surrounds a material in the contactless manner. In an alternate embodiment of the float zone refining process with a contactless heater, the rod is stationary and the heater moves vertically across the length of the rod. In addition to the translational motion, a rotational motion may also be imparted to improve crystal perfection and uniformity. The float zone process with a contactless heater, while producing a clean monocrystalline result, is very unstable in that the melt zone tends to collapse.
Variation of crystal diameters is a significant problem in crystals made by the conventional Czochralski method. Sophisticated control systems have been proposed, but are limited in that only round crystals can be grown.
The Czochralski method has been modified by use of shapers in the crystal producing apparatus, to overcome some of the problems identified in the basic, or conventional Czochralski method.
Floating-shaper methods have used to grow Ge and Si crystals using a floating cover or crucible of graphite. See, e.g., U.S. Pat. Nos. 3,002,824; 3,291,574; and 4,167,554. Typically the shaper temperature is not controlled. To keep a crystal diameter uniform, a laser beam may be shone on the meniscus to detect variations in the diameter of the crystal. If the crystal diameter increases, the melt temperature or pulling speed is increased to reduce crystal diameter. (See U.S. Pat. No. 3,291,650.) Limitations include complications due to lowering of the melt level. It does not work well if the melt is encapsulated or transparent.
The Ohno continuous casting method (OCC) has been used for a number of metals and alloys. See, e.g., U.S. Pat. No. 4,515,204. In the Ohno method, the mold is heated with an internal heater to slightly above the melting point of the crystal material. Horizontal and downward casting were used for smaller cross-sections, in which melt break out can be avoided. Limitations are the need for careful adjustments to keep the top of the shaper even with the melt levels at all times. The melt must not wet the mold.
Shaped crystal growth technology is an active area in growth of crystals for use in electronic, optical, and structural materials. See, e.g., Proceedings of Shaped Crystal Growth, 1980, 1987, 1989, North Holland Press.
Most dopants in single crystals have an equilibrium segregation ratio (k) not equal to unity and thus have a natural tendency to segregate along the crystals. Dopant segregation can be aggravated by convection in the melt during crystal growth, which in the case of Czochralski pulling is induced mainly by gravity and crystal rotation.
Several techniques have been used to try to control dopant segregation in Czochralski pulling. In the push-pull technique, feeding must be kept at the same rate as pulling, which is complicated by the fact that the crystal diameter tends to vary during pulling. Also, to accommodate both the feed rod and the growing crystal in a single crucible, the crucible diameter is nearly doubled, thus promoting gravity-induced natural convection in the melt. A two-crucible version of the push-pull technique has been used, one crucible for feeding and the other for pulling. In a similar technique, i.e., the drop-pull technique, an immersed partition cylinder is used to separate the crucible into two regions, the inner one for pulling and the outer one for dropping the feed powder. In the floating-crucible technique, the bottom of the floating crucible is connected to a weight under the outer crucible, with a connecting shaft which goes through the bottom of the main crucible and melt in it. It is worth noting that electromagnetic fields, which can only be applied to current-conducting melts, can help reduce dopant segregation by reducing convection in the melt. However, as long as k.noteq.1, dopant segregation will occur even in a completely convectionless state. This situation is particularly bad if k is far from unity.
The present invention addresses the problems in producing crystals of uniform diameter and dopant distribution.